Theme export: Stop slugs being cast to ints when theme is exported

[glendaviesnz]

What?

Prevent slugs containing only digits from being cast to ints by PHP when used as keys in theme.json processing

Why?

Fixes one of the issues noted in #44546 that spacing preset slugs are cast to ints when a theme is exported via the site editor

How?

Cast the array key to a string when converting back to the preset sizing array

Testing Instructions

• In a site with the 2023 theme set export the theme from the site editor
• Make sure the exported theme.json has the settings.spacingScale slugs as strings
• Check that the spacing presets still work as expected in post editor, site editor and frontend

Screenshots or screencast

before:

after:

[glendaviesnz]

PR to port to core at WordPress/wordpress-develop#3371

[ramonjd]

Slugs LGTM

"spacing": {
	"spacingScale": [],
	"spacingSizes": [
		{
			"name": "1",
			"size": "clamp(1.5rem, 5vw, 2rem)",
			"slug": "30"
		},
		{
			"name": "2",
			"size": "clamp(1.8rem, 1.8rem + ((1vw - 0.48rem) * 2.885), 3rem)",
			"slug": "40"
		},
		{
			"name": "3",
			"size": "clamp(2.5rem, 8vw, 6.5rem)",
			"slug": "50"
		},
		{
			"name": "4",
			"size": "clamp(3.75rem, 10vw, 7rem)",
			"slug": "60"
		},
		{
			"name": "5",
			"size": "clamp(5rem, 5.25rem + ((1vw - 0.48rem) * 9.096), 8rem)",
			"slug": "70"
		},
		{
			"name": "6",
			"size": "clamp(7rem, 14vw, 11rem)",
			"slug": "80"
		}
	],

Spacing scale absence will be fixed in #44555 I take it